23 research outputs found
Simulations of galactic dynamos
We review our current understanding of galactic dynamo theory, paying
particular attention to numerical simulations both of the mean-field equations
and the original three-dimensional equations relevant to describing the
magnetic field evolution for a turbulent flow. We emphasize the theoretical
difficulties in explaining non-axisymmetric magnetic fields in galaxies and
discuss the observational basis for such results in terms of rotation measure
analysis. Next, we discuss nonlinear theory, the role of magnetic helicity
conservation and magnetic helicity fluxes. This leads to the possibility that
galactic magnetic fields may be bi-helical, with opposite signs of helicity and
large and small length scales. We discuss their observational signatures and
close by discussing the possibilities of explaining the origin of primordial
magnetic fields.Comment: 28 pages, 15 figure, to appear in Lecture Notes in Physics "Magnetic
fields in diffuse media", Eds. E. de Gouveia Dal Pino and A. Lazaria
Observational Constraints on the Common Envelope Phase
The common envelope phase was first proposed more than forty years ago to
explain the origins of evolved, close binaries like cataclysmic variables. It
is now believed that the phase plays a critical role in the formation of a wide
variety of other phenomena ranging from type Ia supernovae through to binary
black holes, while common envelope mergers are likely responsible for a range
of enigmatic transients and supernova imposters. Yet, despite its clear
importance, the common envelope phase is still rather poorly understood. Here,
we outline some of the basic principles involved, the remaining questions as
well as some of the recent observational hints from common envelope phenomena -
namely planetary nebulae and luminous red novae - which may lead to answering
these open questions.Comment: 29 pages, 8 figures. To appear in the book "Reviews in Frontiers of
Modern Astrophysics: From Space Debris to Cosmology" (eds. Kabath, Jones and
Skarka; publisher Springer Nature) funded by the European Union Erasmus+
Strategic Partnership grant "Per Aspera Ad Astra Simul"
2017-1-CZ01-KA203-03556
Increasing the effectiveness of the Diabetes Prevention Program through if-then plans: study protocol for the randomized controlled trial of the McGill CHIP Healthy Weight Program
A new constraint on mean-field galactic dynamo theory
Appealing to an analytical result from mean-field theory, we show, using a
generic galaxy model, that galactic dynamo action can be suppressed by
small-scale magnetic fluctuations. This is caused by the magnetic analogue of
the R\"{a}dler or effect, where rotation-induced corrections
to the mean-field turbulent transport result in what we interpret to be an
effective reduction of the standard effect in the presence of
small-scale magnetic fields.Comment: 6 pages, 2 figures, 1 table, edited to match MNRAS versio